Tunable lasers are used, for example, in the field of optoelectronics and in spectroscopy. In this context, efforts have been directed, in particular, to providing compact, narrow-band laser light sources having rapidly tunable emission wavelengths in the visible spectral region.
At present, this task is mainly fulfilled by the following approaches: dye lasers having a variable external resonator, frequency tripling of the emission of a broadband infrared-emitting laser or white light generation using femtosecond laser pulses.
Dye lasers having a variable external resonator are described, for example, in U.S. Pat. Nos. 3,959,739 and 4,028,636. As an active medium, these laser systems use a fluorescent dye that has been dissolved in a solvent and is optically excited.
International Application Publication No. WO 2004/077142 describes an infrared radiation source which realizes tunable emissions within a specific wavelength range by tripling the frequency of the emission. In addition, it is known from the related art that, by tripling the emission frequency of an erbium fiber laser, it is possible to set a laser emission that is tunable between 495 nm and 650 nm. The frequency tripling is accomplished with the aid of an optical parametric oscillator (OPO). The thermal load on the OPO is reduced by the use of ultrashort-pulse laser systems. In principle, however, the ultrashort-pulse laser systems are precluded from attaining the narrowband characteristic of dye laser systems because of the bandwidth limit. Moreover, the wavelength selection is performed by mechanically adjusting the resonator.
Another approach known from the related art is based on the generation of white light using femtosecond laser pulses, as discussed in the International Application Publication No. WO 2005/071483. Besides the femtosecond laser, these systems also require a special nanostructured optical fiber for generating the white light spectrum, which makes them very complex, both mechanically and optically. In these systems, the emission wavelength can only be selected by filtering out the unwanted spectral components.
All of the approaches mentioned are based on changing the parameters of the optical resonator. Since this must be carried out with exceptional precision, these systems are very complex and place stringent demands on mechanical stability. At present, however, narrow-band laser emission is only attainable through the use of dye lasers.